1. Field of the Invention
The present invention relates to a method of forming a ruthenium thin film, and more particularly, to a method of forming a ruthenium thin film using a plasma enhanced process.
2. Description of the Related Art
A dielectric thin film of a conventional NO or ONO structure has a limitation if it is to be applied to a next-generation memory device. Therefore, many researches on high dielectric thin films have been actively made and result in the appearance of Ta2O5 layer, lead zirconate titanate (PZT) layer, barium strontium titanate (BZT) layer, etc. Accordingly, capacitor""s electrode materials adapted to the corresponding high dielectric thin films have been also researched. Among them, one is a ruthenium (Ru) thin film. Typically, the ruthenium thin film is deposited using a metal-organic chemical vapor deposition (MOCVD) process, which uses a tris(2,4-octanedionato)Ru (hereinafter, referred to as Ru(OD)3) as a ruthenium precursor. At this time, it is very difficult to secure processing conditions capable of simultaneously satisfying an electric conductivity, a smooth surface morphology, an excellent step coverage, etc.
Generally, the Ru(OD)3 is kept in a state that it is dissolved in methanol, and it is evaporated and supplied into a reaction chamber together with an oxygen gas and an argon gas to thereby deposit the ruthenium thin film on a semiconductor substrate-previously mounted within the reaction chamber. At this time, with respect to an oxygen fraction, a pressure, a temperature, etc., conditions for improving the surface morphology are contrary to those for improving the step coverage.
FIGS. 1 and 2 are a scanning electron microscope (SEM) photograph and an atomic force microscope (AFM) photograph of a surface of the ruthenium thin film deposited using a thermal chemical vapor deposition (CVD) process under the condition for improving the surface morphology, respectively. To the satisfaction, as can be seen from FIGS. 1 and 2, the surface morphology (rms) is approximately 1.9 nm. Under this condition, however, the step coverage is not good. Meanwhile, if the oxygen fraction is increased, the step coverage is improved, as shown in FIG. 3. However, the surface morphology is rapidly increased to approximately 6.9 nm. Therefore, in case where the ruthenium thin film is used as a bottom electrode of the dielectric thin film, a phenomenon of local electric-field concentration may occur, so that an electric property of the dielectric thin film is degraded.
As described above, according to the conventional method of forming the ruthenium thin film, since the conditions for improving the surface morphology are contrary to those for improving the step coverage, it is difficult to obtain the thin film having the improved properties in both the surface morphology and the step coverage.
Therefore, the present invention has been devised to solve the above problems, and it is an object of the present invention to provide a method of forming a ruthenium thin film having improved properties in both a surface morphology and a step coverage.
In accordance with an embodiment of the present invention, there is provided a method of forming a ruthenium thin film, which comprises the steps of: a) forming a ruthenium seed layer on a semiconductor substrate using a PECVD process; and b) forming a ruthenium thin film on the ruthenium seed layer using a thermal CVD process.
In accordance with another embodiment of the present invention, there is provided a method of forming a ruthenium thin film, which comprises the steps of: a) forming a ruthenium thin film on a semiconductor substrate using a thermal CVD process; and b) processing a surface of the ruthenium thin film using an oxygen containing plasma.
In accordance with further another embodiment of the present invention, there is provided a method of forming a ruthenium thin film, which comprises the steps of: a) forming a first ruthenium thin film on a semiconductor substrate using a thermal CVD process; and b) forming a second ruthenium thin film on the first ruthenium thin film using a PECVD process.